Polymer materials formed by emulsion polymerization may be processed into powder in order for reduction in volume, a variety of applications, and easy handling. To obtain polymer materials formed by emulsion polymerization into powder, latexes formed by emulsion polymerization require coagulation, dehydration, and drying.
Coagulation of an emulsion-polymerized polymer latex may be performed by breaking stability of latex particles stabilized by an emulsifying agent added during emulsion polymerization by a chemical method using various coagulants or by a mechanical method using mechanical force through application of strong shear stress. In the chemical method, stability is broken using different coagulants according to kinds of emulsifying agents used to secure stability of latexes. In the mechanical method, strong shear stress is applied to latex so that latex particles agglomerate with each other by overcoming repulsive force between emulsifying agents.
As a method of preparing a polymer latex powder, rapid coagulation is proposed. Rapid coagulation is a process whereby polymer particles in a latex rapidly agglomerate with each other by breaking stability of an emulsifying agent through addition of an excess of an aqueous solution of a coagulant such as an inorganic salt, an acid, or the like. Such agglomeration of polymer particles of a latex is referred to as coagulation, and a suspension of these agglomerated polymer particles is referred to as a slurry. The polymer particles are physically weakly combined with each other and thus are easily broken up by external shear stress using a stirrer or the like. Thus, primarily coagulated slurry is subjected to aging whereby binding capacity is enhanced by chain crosslinking through temperature increase. The resulting slurry is subjected to dehydration and drying, thereby completing preparation of a powder-type polymer latex.
In rapid coagulation in which coagulation is performed using an excess of a coagulant, latex stability is very rapidly broken and thus coagulation of polymer latex particles occurs very fast and irregularly. Due to such irregular coagulation, apparent specific gravity is reduced and particle size distribution of finally obtained particles is very wide.
FIG. 1 is a view illustrating a conventional apparatus for preparing a polymer latex resin powder. The apparatus includes a latex storage tank 1, a coagulation bath 2, an aging bath 3, a dehydrator 4, and a dryer 5.
First, a polymer latex stored in the latex storage tank 1 is introduced into the coagulation bath 2 via a polymer latex introduction line 11, and an aqueous coagulant solution and water for adjustment of the concentration of solid content are added to the polymer latex in the coagulation bath 2 via a coagulant introduction line 12 and a water supply line 13, respectively. The added aqueous coagulant solution serves to break electrostatic stabilization obtained by an emulsifying agent and thus enables polymer particles in a latex to agglomerate with each other, thereby obtaining a polymer slurry. The coagulated polymer slurry is transferred to the aging bath 3 and then subjected to aging at high temperature for a residence time of 40 to 90 minutes. The finally obtained slurry is subjected to dehydration in the dehydrator 4 and dried in the dryer 5 to obtain a polymer powder, and the polymer powder is discharged via a polymer powder discharge line 15. A coagulant added to the dehydrator 4 is discharged via a coagulant discharge line 14.
However, when such an apparatus is used, it is difficult to stir and transfer a slurry with high viscosity and thus efficiency of processing into powder is deteriorated. Thus, it is difficult to use a slurry with high solid content in order to increase dehydration and/or drying efficiency and thus only a slurry with low solid content may be used in the apparatus and, accordingly, subsequent dehydration and drying processes are time-consuming and require much effort and energy.
To address the problems described above, a multi-step continuous coagulation and aging process is proposed. This process is advantageous in that a polymer slurry with low solid content may be effectively aged. However, this process cannot be applied to a slurry with high solid content and requires several steps, which results in somewhat low manufacturing efficiency.
In addition, slow coagulation whereby powder characteristics of finally obtained particles are enhanced by adjusting a coagulation rate through divisional introduction of a coagulant is proposed. In this process, coagulation occurs in a secondary well region having energy barrier and thus a coagulation rate is slow and there is a space for rearrangement of particles and, as a result, it is possible to prepare spherical particles by regular filling. However, a total amount of a coagulant used is similar to that used in rapid coagulation and only a difference between slow coagulation and rapid coagulation is that slow coagulation is performed through divisional introduction of a coagulant. Thus, production of waste water due to an excess of a coagulant is inevitable and a smaller amount of a coagulant than that used in rapid coagulation is introduced into a primary coagulation bath and thus the viscosity of a slurry is increased and, accordingly, water needs to be added more than in rapid coagulation in order to secure fluidity. In addition, slow coagulation requires higher moisture content than rapid coagulation.
In both rapid coagulation and slow coagulation, fluidity of a polymer latex slurry prepared after coagulation is affected by solid content, particle size distribution of the slurry, the amount of occluded water of the slurry, and the like and, in particular, largely affected by solid content. When the solid content of the slurry is a certain degree or more, the fluidity of the slurry is dramatically deteriorated and thus the slurry forms a lump and, accordingly, apparatus operation is impossible. Thus, to achieve smooth fluidity of the slurry, an excess of water needs to be further added in coagulation. Addition of water in an excessive amount increases energy cost generated when the slurry is raised to coagulation temperature and aging temperature and causes production of an excess of waste water even in a dehydration process, leading to increased post-treatment costs. In addition, it is impossible to directly use steam and steam is condensed into water as a medium to transmit energy to the slurry, which results in reduced efficiency.
As another method of recovering powder from emulsion-polymerized latex, a method of recovering a polymer latex powder using a gas-phase spray system is used. This method is a process in which a polymer latex with high solid content is sprayed onto a surface on which an excess of a coagulant flows using an atomizer to coagulate the polymer latex and the coagulated polymer latex is recovered into powder. In this case, the polymer latex contacts the coagulant while maintaining high solid content and thus is rapidly coagulated upon contacting the coagulant and, accordingly, high apparent specific gravity is obtained and it is possible to recover spherical powder that may be obtained in slow coagulation. However, an excess of a coagulant has to be used for complete reaction and thus generation of wastewater in an excessive amount is inevitable and clogging of an atomizer frequently occurs, which results in low stability of manufacturing processes.
In addition, there are studies regarding adjustment of a particle size of a powder in the presence of an organic solvent and improved apparent specific gravity, but the powder characteristics as described above may be obtained only when excess organic solvent is used.
As another method, there is shear coagulation in which a slurry is prepared by agglomerating particles in a latex through application of shear stress as strong mechanical force. This method does not require a coagulant and, in shear coagulation, a polymer latex slurry is prepared by applying shear stress by high-speed rotation of 4,000 rpm or more. However, in an emulsion-polymerized polymer latex that secures stability due to use of an emulsifying agent, the emulsifying agent remains in the recovered powder, which adversely affects thermal stability and color in processing.
Therefore, there is still a need to develop an apparatus for preparing a polymer latex resin powder, in which drying efficiency may be enhanced by reducing a moisture content of a slurry and a polymer latex resin powder having excellent color and powder characteristics may be prepared by reducing the amount of a coagulant and a method of preparing a polymer latex resin powder using the same.